Process for removing alkali contamination from electric lamps



Feb. 4, 1969 J. F. WAYMOUTH 3,425,767

PROCESS FOR REMOVING ALKALI CONTAMINATION FROM ELECTRIC LAMPS FiledApril 20, 1967 JOHN E WAYMOUTH INVENTOR W Isa W,

ATTORNEY United States Patent 8 Claims ABSTRACT OF THE DISCLOSURE A lampis exhausted, filled and sealed with an extra length of exhaust tubeattached to the lamp envelope. The lamp is then heated and a D.C.potential applied between the exhaust tube and a lead-in wire, wherebythe contaminating alkali metals are attracted to the exhaust tube anddeposited therein. The lamp is then tipped off adjacent to the envelopeand the exhaust tube, containing the contaminating metal deposit, isdiscarded.

Background of the invention lamp envelope, the tungsten filament and thelead-in wires.

In operation, these contaminants can combine with the halogen fill toform gaseous compounds. In the region of the hot filament, the moleculesof these compounds appear to dissociate and form ions of the alkalimetals. The voltage gradient that exists between the ends of thefilament influences and accelerates the ions, resulting in gaseousionization current flowing between the ends of the filament. Thisphenomenon is known a currentcreep. As the gaseous ionization currentgradually increases to a critical limit, it results in an are whichmelts the ends of the filament and causes premature failure of the lamp.

Summary This invention discloses a novel process for substantiallyeliminating premature lamp failures resulting from current-creep byconsiderably reducing the amount of alkali metal contamination withinthe lamp. Since the amount needed to cause the above-mentioned gaseousionization current is very small, in the order of fractions of amicrogram, it is neither economically feasible nor sufficient to merelyremove the alkali from the components prior to manufacture of the lamp.For example, the high-silica glass used in the lamp envelope normallycontains about to 100 parts per million of alkali metal impurities andthe tungsten filament contains about to 200 parts per million.Therefore, for an envelope weighing about 3 grams and a filamentweighing about 0.5 gram, the total alkali impurity can vary from aboutto 400 micrograms and it would be prohibitively expensive to purify thecomponents to a total impurity of less than about 1 microgram. It isalso likely that contaminating alkali is introduced into the lamp duringthe manufacturing process from, for example, the apparatus used inpositioning and welding the filament on the support or from theequipment used in sealing the lead-in wires in the high silica glass.

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In my new process, the alkali contaminant is attracted to and depositedon a part of the lamp where it can be easily removed, such as alengthened exhaust tube. Specifically, the lamp is heated until thealkali metals are thermally ionized and, by means of a D.C. potential,are attracted to the exhaust tube. The lamp is then tipped off adjacentto the envelope and the exhaust tube, which now contains the alkalicontaminants, is discarded. Thus the alkali contaminants are reducedsufficiently so that failures due to current-creep caused by the alkaliions are substantially eliminated.

Brief description of the drawing FIGURES 1 and 2 are elevational viewsof a lamp as it is processed in accordance with this invention.

FIGURE 3 is an elevational view of a finished lamp.

Description of the preferred embodiments In FIGURE 1 a lamp is shownwhich has been outgassed, flushed, filled with an inert gas and halogen,and sealed at tip 1 of exhaust tube 2. The exhaust tube is attached toenvelope 3 and is long enough to provide for the subsequent steps of theinvention. A metal clip 4 is disposed around a portion 12 of exhausttube 2 adjacent to envelope 3 and is connected to the negative terminal5 of a D.C. supply 6. One of the external lead-in wires 7 is connectedto the positive terminal ll of D.C. supply 6. The lead-in wires 7 areconnected to internal lead-in wires 14 and the filament 9 through seal10. With filament 9 energized to light the lamp, a D.C. potential isapplied between metal clip 4 and lead-in wire 7 which effectivelyestablishes the potential between tube portion 12 and filament 9.Subsequently, the process is discontinued and metal clip 4 is removed. Adiscoloration 16, usually brown or yellow, is apparent in tube portion12.

As shown in FIGURE 2, the lamp is then tipped off between tube portion12 and envelope 3, for example, by heat-sealing in a manner usual in theart, without exposing the internal fill gas in envelope 3 to theatmosphere, and the contaminant-containing exhaust tube 2, after beingthus detached from lamp envelope 3, is discarded. The finished lamp nowsealed by tip 13 i shown in FIG- URE 3.

The source of the alkali metal contaminant can be lamp envelope 3,filament 9 and internal lead-in wires 14 in addition to thepreviously-mentioned processing equipment. Lamp envelope 3 is usuallymade of high-silica glass and, inherently, contains trace quantities ofalkali metal including sodium, potassium and lithium varying from about5 to parts per million. Filament 9 and internal lead-in wires 14 areusually made of tungsten and contain about 5 to 100 parts per million ofalkali metal. Within a lamp which has been processed and sealed, but notyet electrolyzed by my process, the alkali can exist in the gaseousfill, on the surface of the lamp parts and in solution in the lampparts. It is probable that the reaction where A is the alkali metal andH is the halogen, indicates the mechanism by which the alkali isremoved. When the negative D.C. potential is applied to the outersurface of tube portion 12, the positive alkali ions on its insidesurface are attracted to the outside, leaving the inside surfacedeficient in alkali ions. The alkali halide molecules in the gas cominginto contact with this surface are dissociated, according to reaction 1,and the alkali ions are removed as rapidly as they are formed bydissolving in the silica glass and being attracted toward metal clip 4.The negative halide ions travel toward positive filament 9 and aredischarged there. As the alkali halide content of the gas isprogressively reduced, the alkali ions on all other inside surfaces ofthe lamp react with the halogen fill to form alkali halide molecules,according to reaction 1. These molecules continue to diffuse toward tubeportion 12 where the alkali continues to be removed. As the alkali ionson the inside surfaces are removed, the more deeply embedded ions insolution migrate toward the surface and continue the depletion process.It is usually not necessary to continue the electrolyzing process beyondthe point where a spectrometric analysis of the gas content of the lampdoes not detect any alkali metal.

In the specific example of a 650 watt, 120 volt, T4 lamp a microammeterwas installed in the circuit to measure the electrolysis current andspectrometric techniques were used to measure the amount of alkaliimpurity in the gas phase of the lamp. The limit of detectability ofalkali by this technique appeared to be about 2.1 10- moles. When thislamp was operated at its rated voltage and 75 0 volts D.C. appliedbetween clip 4 and one of lead-in wires 7, a DC. current of positiveions was drawn outward through the silica of from 3 to microamperes.This current would rapidly decrease and a concurrent measurement of thealkali in the gas phase of the lamp showed a similar decrease. When thedetectability limit of alkali was reached, there was still a smallamount of current flow as indicated by the microammeter, probably due inpart to a leakage current in the quartz itself. In eight lamps testedthe total quantity of electrolyzed alkali which had to be removed fromeach lamp to reduce it below the detectability limit of about 2.1x 10-moles was equivalent to between 7.5 and 12 microampere-minutes ofionization current. It is estimated that this is equivalent to atransport of about 5.4 to 8.6 10 moles of alkali.

The reason that the electrolysis current is initially high in thisprocess is probably due to the alkali contaminants initially present inthe gas phase and on the inside lamp surfaces, which are rapidly removedand deposited in exhaust tube portion 12. Contaminants more deeply insolution in the lamp parts or on cooler portions of the lamp are removedat a slower rate, in accordance with reaction 1, and the electrolysiscurrent correspondingly decreases.

The amount of alkali removed in order to eliminate current-creepfailures was small compared to the inherent impurities in the silica andtungsten. In the example of the 650 watt lamp, the 5.4 to 8.6 1O molesof alkali removed is equivalent to 0.12 to 0.2 microgram of sodium. Thesilica envelope weighed 2.2 grams and the tungsten filament weighed0.475 gram. At alkali impurity levels of approximately 8 and parts permillion respectively, this equals 17.2 and 19 micrograms of alkaliimpurity. However, a large proportion of this alkali was volatilized andremoved by the high temperatures during sealing, filament light-up andoutgassing and some of it was deposited on the cooler parts of the lampwhere it could eventually contribute to the current-creep problem is notremoved by electrolysis.

In another test on a series of lamps where the sodium content of the gaswas spectroscopically measured, the average content was 0.13 micrograminitially and less than .003 microgram after electrolysis. Between 0.3and 1.5 micrograms of sodium were removed from each lamp. During lifetesting, none of these lamps showed any evidence of current-creep. Bycontrast, similar lamps made without the electrolysis treatment showedan increase in the average sodium content of the gas to 0.26 microgramafter a few hours of life testing. Tests have shown that a gas sodiumcontent of about 0.3 to 0.4 microgram will result in arc failure of thelamp when it is operated at its rated voltage.

The rate of removal of the contaminating alkali metals can be increasedby operating the lamp at a higher temperature, such as at of its ratedvoltage, during the electrolysis process, or by increasing the DC.potential to about 1500 volts. Electrolysis currents of up to 400microamperes have been measured at these conditions.

The significant improvement in lamp life resulting from this inventionis illustrated by a comparison test of a T4 single-end, hi gh-silicaglass, bromine and argon filled lamp, which is life rated for 25 hoursat volts. The electrolized lamps had an average life of 39 hours, asagainst 3.8 hours for those not electrolized.

Individual spectrometric measurements of the content of sodium,potassium and lithium in the gas during the electrolyzing process showeddecreases in the amount measured that corresponded with the decrease inthe ionization current. On the T4 lamp, electrolized at 750 volts D.C.while the lamp was operated at 120 volts, all three alkali metals wereremoved to undetectable amounts within three minutes.

Although only the preferred embodiment has been described, the essenceof my invention is the application of the DC. potential between twoparts of a lamp whereby the alkali contaminants are attracted to thenegative part and can be effectively removed from contributing to theionization current or current-creep that can occur during the life ofthe lamp. It is apparent that the negative part of the lamp need not bethe exhaust tube but can be any part capable of removal withoutdetriment to the lamp. Other tests have indicated that the contaminantsneed not even be removed from the lamp but can be deposited on arelatively cool portion where the temperature during operation isinsufiicient to result in their thermal ionization. This feature wouldbe dependent on the size of the lamp envelope, the filament temperature,the fill pressure and other lamp characteristics. As indicated above,the rate of removal of the alkali contaminants from the lamp duringelectrolysis is dependent on the DC potential and the temperature towhich it is subjected. On the T4 lamp, the envelope temperature is about750 to 850 C. when the lamp is operated at its rated 120 volts. When theenvelope temperature is increased, either by external means or byoperating the filament at higher than 120 volts, the rate of removal isalso increased. The maximum temperature is limited by the melting pointof the high-silica glass envelope, about 1500 C. Below about 500 C., itis estimated the rate of removal is too slow to be practical.

It is apparent that modifications and changes can be made within thespirit and scope of the instant invention, but it is my intention to belimited only by the appended claims.

What I claim is:

1. In the manufacture of an electric lamp, the steps for removing traceamounts of alkali metal impurities which comprise:

applying a negative DC. potential to a detachable part of the lampenvelope, whereby said alkali metal impurities are attracted to saiddetachable part;

and detaching said detachable part containing said impurities from saidlamp envelope.

2. The process according to claim 1 wherein said detachable part is theexhaust tube.

3. The process according to claim 2 wherein said lamp is heated whilesaid DC. potential is applied.

4. In the manufacture of an electric lamp the steps for removing traceamounts of alkali metal impurities which comprise:

heating said lamp; applying a DC. potential between a lead-in wire and aportion of the exhaust tube of said lamp, said portion to which said DC.potential is applied being negative, whereby said alkali metalimpurities are attracted to said portion; detaching said portioncontaining said impurities from said lamp. 5. The process according toclaim 1 wherein said heat is generated by lighting said lamp.

6. The process according to claim 2 wherein said lamp is filled with ahalogen and an inert gas.

7. In the manufacture of an electric lamp containing a gaseous fillincluding halogen, the steps for removing trace amounts of alkali metalimpurities which comprise: disposing a metal conductor around theexhaust tube of a lamp envelope; lighting said lamp; applying a DC.potential between said conductor and a lead-in wire of said lamp, saidconductor being negative, whereby said alkali metal impurities areionized and attracted to said exhaust tube and are therein collected;sealing said exhaust tube between said conductor and said lamp envelopeand detaching said exhaust tube wherein said alkali metal impurities arecollected. 8. In the manufacture of an electric lamp containing traceamounts of alkali metal impurities, the steps which comprise:

heating said lamp;

applying a negative DC. potential to a part of the envelope of saidlamp, whereby said impurities are attracted to said part of saidenvelope;

the temperature of said part of said envelope, during normal operationof said lamp, not exceeding that at which thermal ionization of saidalkali metal impurities occurs.

References Cited UNITED STATES PATENTS 3,005,674 10/1961 Fraser 316-22RICHARD H. EANES, JR., Primary Examiner.

